There exists in bacteriophage T4 exceptional phage particles that carry a tandem duplication of a segment of the genome that includes the rll region. Our principal efforts will be to further characterize these duplications by genetic and physical methods and to use them as tools to investigate questions of more general nature: (A) Genetic and physical experiments will be performed to elucidate the rules which govern the recombinational behavior of tandem duplications, since the stability of a duplication is determined by these same rules. (B) Mutations in duplicate copies of a gene or of a particular site in a T4 tandem duplication are not independent events. Experiments are proposed to determine if normal recombination (following the replication of a new mutation) is responsible for this apparent anomaly. (C) Duplications are an evolutionarily and developmentally important means of increasing an organism's DNA content. T4 will be used as a model organism to study the formation of duplications. (D) Virtually no conditional lethal mutations have been found in several regions of the T4 genome whose aggregate length is about one half the physical map. The information in these regions is apparently dispensible under standard laboratory growth conditions. We hope to investigate the structure and function of these regions through the characterization of compensating deletions which correct the otherwise increased length of duplication genomes.